THE LATER EMBRYONIC DEVELOPMENT. 



67 



CH ^ 



(Fig. 33) they acquire the >-like shape so characteristic of the 

 adult (Fig. 11, n). 



Th e_ walls of the somites soon undergo important change s. 

 At the time of separation from the archenteron (Fig. 29, ms) 

 the wall of each somite consists of a single layer of cells 

 somewhat irregular in shape and size, but showing no marked 

 differences in different parts. Asjhe somites extend down the 

 s ides of the body they bec om e 

 s omewhat triangular in tran s- 

 vferse section. In each somite 

 there may now be distinguished 

 (Fig. 32) a n outer or parieta l 

 wall, ae xt the external epiblast ; a 

 visceral w all, in contact with the 

 hypoblast of the archenteron ; 

 and a notochordal wall, forming 

 the base of the triangle, and in 

 contact with the notochord and 

 the nerve cord. The cells of th e 

 p arietal and yiscer al walls are 

 alip- htlv flattene d, put show no 

 special peculiarities ; those of the 

 , no tochordal wall, on the oth er 

 hand , show marked changes . 

 Each cell (Fig. 32, ml) is much flattened dorso-ventrally, and 

 elongated in a direction parallel to the axis of the embryo 

 (Fig. 31); and is undergoing changes by whic h it becomes con- 

 ve rted into a muscle cell or fibre . This differentiation of muscle 

 cells begins at a stage witn about nine pair of somites, and 

 proceeds rapidly ; the muscles, at a stage with eleven pairs of 

 somites, beginning to contract and cause lateral undulations of 

 the body. The mass of muscle cella^ formed in this way by 

 modi fiiiation of the notochordal wall of a somite, is called a 

 Jiyotome : the myotomes, being formed from the somites, are, 

 like these, arranged segmentally from their first appearance; 

 they increase rapidly in size, and become the great lateral muscles 

 or myotomes of the adult Amphioxus (Fig. 12, x). J]ach 

 muscle cell exten ds the whole length of the somite to which it 

 belongs. 



In" the higher Vertebrates it will be found that the earliest 



F 2 



Fig. 32. — Transverse section through 

 the middle of an Amphioxus 

 embryo with nine pairs of meso- 

 blastio somites, x 435. (After 

 Hatsohek.) 



CH, notochord. I, spinal cord. 

 ML, muscle layer. MS, cavity of 

 uiesoblastic somite. T, mesenteron. 



